Optical information recording media, typically optical disks such as compact disks and video disks, are designed to record information utilizing the presence or absence of small pits (i.e., arranged in an array along circular or spiral "tracks"), wherein reproduction of recorded information is performed by detecting said presence or absence of small pits using an optical pickup and producing an electrical signal corresponding thereto. In some optical disks, recorded information is detected by directing a light beam along a track and monitoring a difference in the reflectance of a small region on account of pigments or phase change, and producing the electrical signal according to this difference.
An information recording/reproducing system using the optical disks described above have such frequency characteristics that the amount of information (stored via the presence or absence of pits or regions) that can be detected is limited by a spatial frequency which is 2 NA/.lambda. at maximum, where NA is a numerical aperture of a lens in a reproducing optical system, and .lambda. is a wavelength of information detecting light. Therefore, any efforts toward increasing a recording density of information on optical disks (e.g., by such techniques as mastering) have been limited by a fact that the only information that can be detected is that within the spatial frequency 2 NA/.lambda..